Performance and microbial response to nitrate nitrogen removal from simulated groundwater by electrode biofilm reactor with Ti/CNT/Cu5-Pd5 catalytic cathode

Abstract

To enhance the removal of nitrate nitrogen (NO3−-N) in groundwater with a low C/N ratio, electrocatalytic reduction of NO3−-N has received extensive attention since its electrons can be directly produced in situ while simultaneously providing a clean electronic donor of hydrogen for denitrifying bacteria. In this study, Ti/CNT/Cu-Pd bimetallic catalytic electrodes with different copper-palladium (Cu-Pd) ratios were prepared by electrodeposition onto carbon nanotube (CNT) using titanium (Ti) plates. The results showed that the NO3−-N conversion rate by Ti/CNT/Cu5-Pd5 electrode was the highest (53.60%) compared with other Cu-Pd electrode ratios because of the combined role of the copper's high NO3−-N catalytic activity and the palladium's high N2 selectivity. A new type of electrode biofilm reactor (EBR) with Ti/CNT/Cu5-Pd5 cathode, biochar substrate was constructed to explore the removal ability of NO3−-N in simulated low C/N groundwater. When the influent NO3−-N concentration was 30 mg/L, under the condition of a 30 mA electronic current and hydraulic retention time (HRT) of 12 h, the removal rate of NO3−-N could reach as high as 78.1 ± 1.2%, and the N2 conversion rate was 99.7%. The horizontal distribution of microbial communities in EBR showed that the denitrification capacity was significantly improved through the electrochemical catalytic reduction of the Ti/CNT/Cu5-Pd5 cathode and the supply of the hydrogen electron donor to autotrophic denitrogenerating microbes such as Anaerobacillus, Thauera, and Hydrophaga. This study provides a new bimetallic catalytic cathode to enhance the removal of NO3−-N in groundwater with a low C/N ratio. Practitioner Points: The Cu5Pd5/CNTs/Ti electrode is beneficial to the adsorption and reduction of NO3−-N to N2. The production of hydrogen electron donors by cathode promoted nitrogen degradation. Activated electrodes together with denitrifying microorganisms contributed to the improved N removal rate.